It is a well known problem, when surveying a given area, to be able to distinguish between friendly and unfriendly forces moving through it. Normally, the area surveyed is scanned either visually or by movement detecting means, e.g., apparatus which detects heat, and the surveying person is thus alerted of that movement takes place within the surveyed area. When friendly forces move through the surveyed area, the surveying person normally identifies them by maintaining radio contact, or by visual signs, when possible. However, identifying moving persons by radio transmission presents severe drawbacks; the radio transmission can be easily picked up by unfriendly forces moving through the area, heavy radio equipment must be carried if voice communication is desired, and if a simple emitted identification signal is used, it can be relatively easily forged.